Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (with no hazardous materials used), higher efficiency, smaller size, and longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, more energy saving, more efficient correlated color temperature (CCT) tunability, and more aesthetic perception in lighting quality have become especially important and need to be well addressed.
In a retrofit application of a linear LED tube lamp to replace an existing fluorescent tube, the lamp is so configured that the light coming out from the LED light sources illuminates a target area directly. The shortcomings are pixilation, glare, and not enough cut-off at vertical angles greater than 80° above the lamp nadir, which cause users' eyes uncomfortable, thus affecting their mood. Similarly, many conventional LED luminaires adopt direct illumination approach and show a poor lighting quality such as hot and dark spots and shadows.
A conventional 2 by 2 feet panel light troffer uses a square thick acrylic plate as a light diffusing medium. LED light sources located at four lateral sides of the acrylic plate illuminate the four sides of the plate, and evanescent light waves exiting from the front face of the acrylic plate further scatter through a plastic diffuser attached to the acrylic plate in the front panel before launching into a target area. In order to increase optical efficiency, the back panel of the panel light troffer is attached with a reflective sheet. However, such panel light troffers have their light opening flushed with T-bar ceiling grids without recess. Thus, occupants in the room can see the whole bare bright area 2 by 2 feet and feel uncomfortable because a direct glare affects their eyes and thus distracts them.
In today's lighting applications, correlated color temperature (CCT) tuning is important. Although consumers demand a tunable CCT such as warm-white at 2,700 K, sun-white, natural-white, or cool-white at 6,200±300 K in lighting to help improve the atmosphere in working, exhibiting, or living areas, there have been very few such lighting products in the troffer and luminaire markets. The LED panel light troffers do not have a proper structure to sufficiently perform spatial color mixing, which makes it difficult to be successful on the market. Instead, manufacturers can generally make an LED troffer using two kinds of phosphor coated white LEDs, one cool white and the other warm white, to mix the light emissions with different ratios to come up with desired color temperatures. Because at the two color extremes, only one kind of LEDs emits the light, such troffers have poor cost efficiency and luminous efficacy. In spite of these disadvantages, the approach is one of several solutions to changing CCT of an LED troffer in general lighting applications. However, the approach needs a proper color mixing scheme to smooth out lighting outputs such that the color hue is consistent within viewing angles.
Other possible color temperature tuning approaches include a white LED at CCT of 6,200±300 K mixed with an LED having a saturated color, featuring high luminous efficacy; a yellow white LED mixed with a red LED; and RGB color mixing, the earliest approach to varying light color, in which white light is perceived where all three additive primaries overlap. Because of low luminous efficacy and difficulty to meet CIE 1931 recommendations for general lighting in solid state lighting products, such as stabilizing a specific chromaticity over time while LED junction temperatures change from ambient temperature to 120° C. or higher due to different thermal dependencies for an individual LED, the RGB approach is seldom adopted as in general lighting applications today. However, in decorative lighting, RGB color mixing is frequently used. By varying the intensities of the individual red, green, and blue light sources, any colors that human eyes can perceive can be obtained. Surely, in all of the above mentioned CCT tuning approaches, many of same or different LEDs need to be used in combination to achieve a required lumen output. Thus uniformity of the resultant CCT light and color hue within viewing angles becomes an issue if the troffer or luminaire used cannot provide adequate light averaging and mixing functions.
Emergency lighting is especially important in this consumerism era. The emergency lighting systems in retail sales and assembly areas with an occupancy load of 100 or more are required by codes in many cities. Occupational Safety and Health Administration (OSHA) requires that a building's exit paths be properly and automatically lighted at least ninety minutes of illumination at a minimum of 10.8 lux so that an employee with normal vision can see along the exit route after the building power becomes unavailable. This means that emergency egress lighting must operate reliably and effectively during low visibility evacuations. To ensure reliability and effectiveness of backup lighting, building owners should abide by the National Fire Protection Association's (NFPA) emergency egress light requirements that emphasize performance, operation, power source, and testing. OSHA requires most commercial buildings to adhere to the NFPA standards or a significant fine. Meeting OSHA requirements takes time and investment, but not meeting them could result in fines and even prosecution. If a building has egress lighting problems that constitute code violations, the quickest way to fix is to replace the existing troffer with a multi-function LED troffer that has an emergency light package integrated with the normal lighting. The code also requires the emergency lights be inspected and tested periodically on site to ensure they are in proper working conditions at all times.
It is, therefore, the manufacturers' responsibility to design an LED luminaire not only with a uniform illumination output but also with an emergency LED module integrated such that after the LED luminaire is installed on a ceiling, the emergency LED module can individually be inspected, without removing the whole luminaire from the ceiling. Such designs can improve lighting quality and greatly reduce lifetime cost of ownership. Currently no manufacturers have adopted such a cost-effective approach in a luminaire used to replace conventional fixtures for fluorescent lamps.